The quality of raw water and finished (treated) water varies considerably throughout the world. For instance, the types and levels of trace metals found in water from household taps can vary significantly from one country to another. The presence of certain trace metals often reduces the effectiveness of laundering aids and causes fabric damage.
Hypochlorite liquid bleaches have found wide commercial acceptance and are commonly used in a variety of household cleaning and laundering products. The effectiveness of hypochlorite bleach compositions is usually not adversely affected by trace metals found in water used in laundering. However, it has been found that the presence of iron and manganese metal ions in water used in laundering causes fabrics to yellow and deteriorate. This phenomenon is particularly pronounced when the bleaching composition is not used in combination with a laundry detergent.
In WO 96/00188 it was speculated that the presence of metal impurities in the washing environment catalyzed the attack of hypochlorous acid on fabrics with the generation of yellow oxidized species. It was believed that the metal ions are adsorbed on fabrics as colored species and catalyzed the degradation of the brighteners absorbed on fabrics. Furthermore, it was said that the metal ions stablized colored pigments of enzymatic stains, such as blood and grass, that cause such stains to "set". Finally, it was said that the metal ions catalyzed the depolymerization of cotton fibers which leads to reduced tensile strength of the fabrics thereby reducing fabric resistance.
To counter the deleterious effects caused by the presence of heavy metal ions in hypochlorite-containing cleaning compositions, the art has suggested the employment of various agents to improve fabric whiteness and integrity. These agents, for example, include periodate, silicates, and pyridine with carboxylic acid substituents. See, for example, WO 96/00188, and EP 0653482, and EP 0653483.
While some of these complexing agents have demonstrated hypochlorite bleach enhancement in terms of fabric whitening and safety, they are not satisfactory for a number of reasons. First, these prior art complexing agents are expensive to use. Second, some of these complexing agents do not mitigate the dingying effect of hypoclorite on fabrics washed in water containing significant levels of iron and/or manganese ions.